The present invention concerns the addition of terminators to reduce radio frequency (RF) transmissions which appear when RF energy propagated on a printed circuit assembly (PCA), due to geometrically induced transmission line effects, propagates outside the PCA and becomes radiated waves. The present invention also concerns the design of connectors on the PCA which are used by the terminators.
When components on PCAs contain digital signals which switch state, excess energy, in the form of RF signals, is generated. The RF signals are generated from the residual spectral energy that is not necessary for the functionality of the digital circuits. Often these RF signals are available in adequate levels to be coupled into potentially radiating structures. This may result in RF frequency pollution being transmitted outside the system containing the PCA.
A slot or gap of any length between two metal surfaces can form a potential transmission line which can serve as a radiating structure which propagates and enhances RF signal matching to the environment. Consequently, it is desirable to somehow reduce the radiating effect of slots or gaps which exist in PCAs and their surrounding structures.
In the prior art, various design features are used to reduce radiated RF signals. For example, in order to filter out the transfer of electrical noise between two planer structures, the two planar structures may be isolated from each other. Signals traveling between the two planar structures are propagated through narrow traces, which are much narrower than the distance between the two planar structures. However, the resultant structures may have resonant properties at certain frequencies which can be within the range of the frequency spectrum of the harmonic frequencies of the functional digital logic used. Such a design, may result in acceptable isolation between the two planar structures until a change in clock frequency or digital logic edge speed will stimulate the structure to resonate with the result of unacceptable RF emissions.
Alternate to the use of small thin traces, a choke coil has been used for connections between two isolated planar structures. However, the Q of the resulting circuit can still be too high to preclude discreet resonances. Thus emissions at selective resonant frequencies may still occur. Additionally, this method requires the use of expensive choke coils, which increase the manufacturing cost of the total system.
Various resistance and capacitance filter schemes using discrete devices have also been employed to decouple RF energy from a product and its outside interfaces. These schemes also can increase the manufacturing cost of the total system.
In addition, in order to eliminate electromagnetic interference (EMI), metal planes have been used that fold back under a PCA and are connected to the PCA along a single edge. However, these metal planes also have a tendency to resonate and radiate RF signals, often at unexpected frequencies.
Also, a well shielded cabinet may be used in order to prevent the escape of RF radiation. While often successful, small changes in the methods of fastening the cabinet or the introduction of air vents may result in increased RF signal emissions. Additionally, well shielded cabinets can be costly, adding to the manufacturing costs of a system.
In a common prior art solution, PCAs are grounded to a sheet metal cardcage by using spring-finger card guides. These guides are usually riveted to the metal cardcage and usually run the entire depth of the PCA, thus providing numerous contact points along the edge of the PCA. Flexible, spring-fingers of the rigidly mounted cardguide make contact with a plated section or pad on the surface of the circuit board. If grounding is not desired along the entire edge of the board, then discrete sections of the guide can be riveted in place and discreet pads are required on the surface of the board. However, rivet-on grounding devices are an expensive and cumbersome way to achieve grounding. Further, riveted-on guides can be hard to repair if a spring-finger is damaged or broken, since rivets would need to be removed and then replaced. Some of these guides can catch or snag the leading corner or edge of a circuit board, unless the guide provides a good, snag-free lead-in feature. In some cases, the edge of the board can actually slide over the head of the rivet that is used to attach the guide. This has the potential for snagging the board, especially if the rivet has not been installed properly. Retrofitting a cardcage with rivet-on grounding devices may be difficult, since the thickness of the guide and the heads on the rivets may not fit in the allowable height. Additionally, spring-finger guides apply a lateral force to the circuit board. This can sometimes cause misregistration between the mating connectors of the backplane and the slide-in circuit board. Insertion forces are always higher when using a spring-finger guide. This increased insertion force is a disadvantage.